In recent years, fiber-optic chemical sensors, sometimes called optrodes, have been developed to detect the presence and monitor the concentration of various analytes, including oxygen, carbon dioxide, and hydrogen ions (i.e., pH) in liquids and in gases. Such sensors are based on the recognized phenomenon that the absorbance, and in some cases luminescence, that is, the phosphorescence or fluorescence of certain indicator molecules, are specifically perturbed in the presence of specific analyte molecules. The perturbation of the luminescence and/or absorbance profile can be detected by monitoring radiation that is absorbed, reflected, or emitted by the indicator molecule in the presence of a specific analyte.
Fiber-optic sensors relying on these characteristics position the analyte sensitive indicator molecule in a light path at a desired measurement site. Typically, the optical fiber transmits electromagnetic radiation from a light source to the indicator molecule, and the reflectance from or absorption of light by the indicator molecule gives an indication of the gaseous or ionic concentration of the analyte. Alternatively, for monitoring other analytes such as oxygen, the optical fiber transmits electromagnetic radiation to the indicator molecule, exciting it into a type of luminescence, for instance phosphorescence, and the level and/or duration of phosphorescence by the indicator molecule serves as an indication of the concentration of that gas in the surrounding fluid. In the prior art sensors, the indicator molecules are typically disposed in a sealed chamber at the distal end of an optical fiber, and the chamber walls are permeable to the analyte of interest.
In view of the importance of accurately measuring blood gas parameters such as carbon dioxide, oxygen, and pH, there is an existing need to provide a fiber-optic sensing system that provides accurate and timely information in actual use. The sensing system should be immune to or compensate for signal artifacts that can be caused by external influences. While prior sensing systems may perform adequately under controlled environments, such as bench scale tests, when animal tests or clinical tests on humans are conducted, the reliability and performance can be noticeably different. Identifying and addressing the sources of changes in the performance of the system when going from bench scale to clinical scale testing will be critical to the development of such sensing systems.